1. Technical Field
The present invention relates to a thin and high brightness LCD (liquid crystal display) backlight used for a non-self emission type display device such as a liquid crystal TV, and a liquid crystal display device using this LCD backlight.
2. Background Art
A liquid crystal display uses a system of displaying images by controlling the transmission quantity of light that is irradiated from the rear face using the electro-optical effect based on liquid crystal molecule orientation, and generally requires a flat illumination device called a “backlight unit” that is constituted by fluorescent display tubes. Recently the screen size of liquid crystal devices is increasing, and now even a 50 inch size or larger TV display device is commercialized. At the same time power consumption is increasing as well as screen size increases, therefore technical developments to implement lower power consumption is demanded, and in order to minimize the space to be occupied in a room where the display device is installed as well, thin construction is strongly demanded. At the same time, an increase in brightness and resolution is demanded.
In order to support lower power consumption and higher brightness, of these demands, the use of light emitting diodes (hereafter LED(s)) and lasers are being considered as light sources having good emission efficiency, and an LCD backlight using an LED as a light source has already been commercialized. However if a laser light source is used to support further lower power consumption, high brightness and a wider color gamut, it is critical to improve dust resistance of the LCD backlight that irradiates the liquid crystal panel from the rear face. Because laser light, which has linear propagation characteristics, can be condensed to a small spot, and this becomes a factor of differentiation when the device is downsized, if micro-dust deposits on the surface of the optical component, then an image quality problem occurs to the image display device, such as a drop in transmittance of the laser light and uneven brightness.
Possible solutions to solve this problem are assembling the optical components of the liquid crystal display device in a clean room where minimal dust exists, or physically sealing the optical system of the liquid crystal display device so that no dust enters from the outside. However these solution are impractical when the necessity of an efficient cooling system for suppressing a temperature rise of the LCD backlight, including the optical system, and the manufacturing cost of the liquid crystal display device are considered.
In an optical system that deflects the laser light to detect signals or to record signals, such as an image recording device, printer and facsimile, it has been proposed to prevent the adhesion of dust to a mirror of which incident angle of a laser light changes due to the deflection of the laser light by installing the mirror such that the surface normal line thereof becomes lower from the horizontal line (e.g. See Patent Document 1). According to this proposal, the adhesion of not only dust but also toner to the mirror can be prevented, and a drop in reflectance and reflection unevenness can be suppressed.
In a multi-color image output device, it has been disclosed that a mirror face of a reflection mirror for guiding light to each exposure unit can be disposed downward by disposing the exposure unit in the upper circumference of a photosensitive body, so as to prevent the adhesion of dust to the mirror face by self weight (e.g. see Patent Document 2). According to this proposal, the reflected light or laser light is not affected and image quality deterioration due to dust on the mirror face does not occur.
In a digital xerography type image formation device, such as a laser printer, it has been disclosed that a drop in light quantity of a light beam on the surface of a photosensitive body, due to dust adhering to the emission window, is confined to within a predetermined ratio by adjusting the distance between the emission window of the laser light and the photosensitive body and a spot diameter of the light beam on the photosensitive body (e.g. see Patent Document 3).
However in the above mentioned prior art, a configuration to deflect the laser light, not in the vertical plane direction, but in the horizontal plane direction for scanning is disclosed, however an optimum configuration to scan with laser light in the vertical plane direction, which can be applied to a thin LCD backlight and liquid crystal display device, is not disclosed in concrete terms.
In other words, a conventional laser printer or the like is not thin and upright, and as a typical layout of the optical system is a device, the laser optical system is disposed on an optical board that is placed approximately horizontal in an enclosure. Therefore the laser light is guided on a same horizontal plane, and is diagonally deflected downward at the end, targeting the photosensitive body, so the normal line directions of the entrance plane and emission plane of the laser light of each optical component inevitably become lower than the horizontal direction. Therefore even if a plurality of laser sources having different wavelengths are used, dust is hardly deposited in this structure.
If an optical system of an LCD backlight is constructed using a laser light, on the other hand, a thin liquid crystal TV can be constructed by disposing an optical system on the rear face of the liquid crystal panel in a state where the optical board is vertically disposed. In this case, dust deposits on the side face of each optical component constituting the optical system, and if dust is deposited on the side face where a laser light transmits, the brightness of the LCD backlight drops caused by the drop in laser light transmittance because of dust, or unevenness brightness occurs generated by a partial eclipse of the laser light due to dust.
In the case of a laser printer, the intensity of the laser light to be used is low, at the milliwatt level, and infrared light is often used, but in the case of a liquid crystal TV, a watt level high power light source is used, and a blue short wavelength light source is included, which causes dust to easily adhere to the surface of the optical component due to induced static electricity thereon.    Patent Document 1: Japanese Patent Application Laid-Open No. S53-142247    Patent Document 2: Japanese Patent Application Laid-Open No. S61-270772    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-337386